Composite strip windable to form a helical pipe and method therefor

ABSTRACT

A composite strip  10  windable to form a helical pipe is disclosed. The composite strip  10  comprises: an elongate plastic strip  11  having a base portion  12  and at least one lengthwise extending rib portion  20  upstanding from the base portion  12;  and an elongate reinforcing strip  30  extending lengthwise and supported laterally by the rib portion  20,  the reinforcing strip  30  having a height to thickness ratio of at least three to one and orientated substantially perpendicular to the base portion  12.  When wound into a helical pipe, the reinforcing strip  30  reinforces the pipe against radial crushing loads. A bead  40  seals the reinforcing strip  30  from the environment. Desirably the composite strip  10  further comprises: a planar lamina  50  extending lengthwise and bonded to the base portion  12,  the lamina  50  having a higher Young&#39;s modulus and strength than those of the plastic strip  11.  The lamina  50  greatly improves the pressure rating of a pipe wound from the strip  10.

FIELD OF THE INVENTION

[0001] This invention relates to improvements to reinforced ribbedstructures, and in particular to reinforced or strengthened helicallywound pipes or tubes made from a composite of materials.

BACKGROUND

[0002] It is well known that plastic pipes can be made by helicallywinding a plastic strip having a series of spaced apart upstanding ribsextending longitudinally of the strip, either at room temperature or atan elevated temperature where the plastic becomes more flexible. Thisform of helically wound tube is already well known in the pipingindustry and is described in Patents by the applicant relating both tothe form of the plastic strip and the form of the machine by means ofwhich the pipes or tubes are produced from such strips.

[0003] For these pipes to perform in a high performance applications, inorder to attain the necessary degree of strength, the wall thickness ofthe plastic strip must be quite substantial, as well as that of theribs. Alternatively the finished pipes or tubes can be reinforced withstrengthening or reinforcing members.

[0004] In applications where the reinforced tubes or pipes are buried ina trench or subjected to high earth loads, the strength of the pipe ortube is of extreme importance.

[0005] The applicant's Australian Patent No. 607431 discloses a methodof producing a reinforced plastics tube utilising a reinforcing memberplaced between the ribs in such a manner that the deflection resistanceof the finished pipe or tube is materially increased. The reinforcingmember comprises a metal member having a profile of U-shapecross-section, the free ends of the reinforcing member being designed toengage beneath opposed flange formations of a pair of adjacent ribs tothereby lock the metal strip in position between the ribs and in turnstiffen the ribs and the finished pipe.

[0006] The applicant's Australian Patent No. 661047 discloses animprovement over the disclosure of Australian Patent No. 607431 referredto above. The improvement is provided by the provision of a reinforcingmember having a central body portion of inverted U or V-shapedcross-section that has a radial height greater than the height of theribs whereby the effective external diameter of the composite pipe issubstantially increased. This provides a stiffer pipe.

[0007] Known helically wound composite pipes are formed in a multi-stageoperation. The plastic body is extruded and then is helically wound toform a pipe. Elongate steel reinforcing members are separatelyroll-formed into a profile providing the required stiffness (such as theinverted U or V-shaped profiles referred to above). The roll formedsteel profile is then rolled to a radius approximating that of thehelically wound plastics body. Finally, the profiled and radiusedreinforcing member or members are wound on to the outside of theplastics pipe to form a composite pipe of the requisite stiffness.

[0008] When using the reinforcing members disclosed in Australian PatentNos 607431 and 661047, the step of rolling the steel reinforcing memberto a radius of approximating that of the plastics pipe involvesstraining the steel reinforcing member beyond its elastic limit. Thisrequires the application of considerable force during the rollingprocess. In contrast, winding of the extruded plastics profile into ahelical pipe generally requires much less force due to the materialproperties of the plastics.

[0009] An object of the present invention is to provide certainimprovements, beyond those disclosed in the aforesaid PatentSpecifications 607431 and 661047, to reinforced helically wound plasticstube or pipe, which are effective to stiffen the tube or pipe throughthe addition of reinforcing members formed of plastics material and/orof metal to thereby produce a composite plastics and metal structure.

[0010] It is another object of the invention to provide an improvedcomposite strip that can be wound formable into a helical pipe or tubewithout the need for the addition of a reinforcing member during orafter the pipe winding process. It is also an object of the invention toprovide a method of producing such a composite strip.

[0011] It is yet another object of the present invention to provide amethod of producing a helically wound pipe that obviates the need forpre-rolling the reinforcing member before it is introduced into theplastics body of the strip.

[0012] It is yet another object of the present invention to provide ahelically wound plastics pipe formed of two or more materials havingdifferent characteristics so that the resultant or finished pipe or tubehas enhanced properties, and which can be produced at relatively lowcost.

[0013] It is yet another object of the present invention to provide ahelically wound plastic pipe with a high pressure rating, that is, apipe that is capable of withstanding high internal pressure withoutfailure.

SUMMARY OF THE INVENTION

[0014] According to a first aspect of the invention there is provided acomposite strip windable to form a helical pipe, the composite stripcomprising:

[0015] an elongate plastic strip having a base portion and at least onelengthwise extending rib portion upstanding from the base portion; and

[0016] an elongate reinforcing strip extending lengthwise and supportedlaterally by the rib portion, the reinforcing strip having a height tothickness ratio of at least three to one and orientated substantiallyperpendicular to the base portion,

[0017] wherein, when wound into a helical pipe, the reinforcing stripreinforces the pipe against radial crushing loads.

[0018] Preferably said height to thickness ratio is at least four toone.

[0019] Preferably the rib portion defines a lengthwise extending slot inwhich the reinforcing strip is retained, the reinforcing strip beinglaterally supported by the walls of the slot.

[0020] Preferably the rib portion comprises a pair of parallel wallsextending lengthwise along the base portion, the parallel wallsorientated substantially perpendicular to the base portion.

[0021] Preferably the reinforcing strip is continuous and has a lengthwhich is co-extensive with the plastic strip.

[0022] Preferably the reinforcing strip is completely encapsulated so asto prevent exposure to the environment.

[0023] Preferably the plastic strip has an array of lengthwise extendingslot forming rib portions spaced apart across the width of the strip,each rib portion supporting an elongate reinforcing strip.

[0024] The composite strip may include various materials, however,preferably the reinforcing strip is constructed from metal. Moreparticularly, preferably the reinforcing strip is constructed fromsteel.

[0025] According to a second aspect of the invention there is provided acomposite strip windable to form a helical pipe, the composite stripcomprising:

[0026] an elongate plastic strip having a base portion and a lengthwiseextending rib portion upstanding from the base portion;

[0027] an elongate planar reinforcing strip extending lengthwise andsupported by the rib portion, the reinforcing strip having a height tothickness ratio of at least three to one and orientated substantiallyperpendicular to the base portion; and

[0028] a planar lamina extending lengthwise and bonded to the baseportion, the lamina having a higher Young's modulus and strength thanthose of the plastic strip,

[0029] wherein, when wound into a helical pipe, the reinforcing stripreinforces the pipe against radial crushing loads and the laminaimproves the pressure rating of the pipe.

[0030] Preferably said height to thickness ratio is at least four toone.

[0031] According to a third aspect of the invention there is provided ahelically wound composite pipe produced from a composite strip, thecomposite strip comprising:

[0032] an elongate plastic strip having a base portion that forms theinside of the wound pipe and a lengthwise extending rib portionupstanding from the base portion; and

[0033] an elongate metal reinforcing strip extending lengthwise andsupported by the rib portion, the reinforcing strip having a height tothickness ratio of at least three to one and orientated substantiallyperpendicular to the flat side of the base portion,

[0034] wherein the orientation of the reinforcing strip with respect tothe base portion remains substantially unchanged after the winding ofthe strip to form the pipe.

[0035] Preferably said height to thickness ratio is at least four toone.

[0036] Preferably the reinforcing strip is continuous and has a lengthwhich is co-extensive with the pipe.

[0037] Preferably the reinforcing strip is constructed from metal.Preferably the metal is steel.

[0038] Desirably the composite strip further comprises:

[0039] a planar lamina extending lengthwise and bonded to the baseportion, the lamina having a higher Young's modulus and strength thanthose of the plastic strip.

[0040] According to a fourth aspect of the invention there is provided amethod of producing a helically wound steel reinforced plastics pipecomprising the steps of:

[0041] extruding a plastics profile having a base portion and alengthwise extending rib portion upstanding from the base portion;

[0042] introducing an elongate straight edged reinforcing strip so as toextend lengthwise of the rib portion and supported laterally thereby,the metal strip having a height to thickness ratio of at least three toone and orientated substantially perpendicular to the flat side of thebase portion, thereby producing a straight composite strip;

[0043] helically winding the composite strip; and

[0044] inter-engaging the adjacent edges of adjacent convolutions of thestrip so as to form a helical pipe.

[0045] Preferably the rib portion is extruded to define a lengthwiseextending slot shaped to receive and support the reinforcing strip.

[0046] Preferably the rib portion is extruded to comprise a pair ofparallel walls extending lengthwise along the base portion, the wallsorientated substantially perpendicular to the flat side.

[0047] Preferably the method further comprises the step of encapsulatingthe reinforcing strip.

[0048] Preferably the extruding and introducing steps occur together ina cross-head extrusion die.

[0049] According to a fifth aspect of the invention there is provided amethod of producing a helically wound steel reinforced plastics pipecomprising the steps of:

[0050] extruding a plastics profile having a base portion and alengthwise extending rib portion upstanding from the base portion; and

[0051] introducing an elongate metal reinforcing strip into the ribportion, the metal strip having a height to thickness ratio of at leastthree to one and orientated substantially perpendicular to the baseportion, thereby producing a composite strip;

[0052] bonding a lamina to the flat side of the base portion, the laminahaving a higher Young's modulus and strength than those of plasticstrip; and

[0053] helically winding the composite strip; and

[0054] inter-engaging the adjacent edges of adjacent convolutions of thestrip so as to form a helical pipe.

[0055] Preferably the method has further steps between the bonding andhelically winding steps, the further steps comprising:

[0056] directing the straight composite step to a spool having a hubrotating about a substantially horizontal axis, with the base portion ofthe strip facing the underside of the hub;

[0057] driving the spool so as to pull the straight composite striptowards the spool and so as to wind the strip around the hub of thespool from its underside;

[0058] transporting the spool to a site; and

[0059] unwinding the strip from the spool.

DETAIL DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0060] Several preferred embodiments of the invention are illustrated inthe accompanying representations in which:

[0061]FIG. 1 shows a cross-sectional view of a composite strip accordingto a first embodiment of the invention.

[0062]FIG. 2 shows an exploded view of the strip of FIG. 1.

[0063]FIG. 3 is a perspective view of the composite strip shown in FIG.1.

[0064]FIG. 4 is a perspective view of a helically wound composite pipewound from the profile shown in FIGS. 1 and 3.

[0065]FIG. 5 shows a part-sectional view of the pipe of FIG. 4 revealingthe reinforcing element.

[0066]FIG. 6 is a perspective view showing the reinforcing element beingintroduced to the profile.

[0067]FIG. 7 shows a cross sectional view of a composite strip accordingto a second embodiment of the invention.

[0068]FIG. 8 shows a cross sectional view of adjacent convolutions of acomposite strip according to a third embodiment of the invention.

[0069]FIG. 9 shows a cross sectional view of a composite strip accordingto a fourth embodiment of the invention.

[0070]FIG. 10 shows a cross sectional view of a composite stripaccording to a fifth embodiment of the invention.

[0071]FIG. 11 shows a cross sectional view of a composite stripaccording to a sixth embodiment of the invention.

[0072]FIG. 12 shows a cross sectional view of a composite stripaccording to a seventh embodiment of the invention.

[0073]FIG. 13 shows a cross sectional view of a composite stripaccording to a eighth embodiment of the invention.

[0074]FIG. 14 shows a perspective view of a spool drive assembly for usewith embodiments of the invention.

[0075] Referring to FIGS. 1 and 2, an elongate composite strip 10 thatis windable to form a helical pipe is shown. The composite strip 10comprises an elongate plastic strip 11 and an elongate metal reinforcingstrip 30. The plastic used for this embodiment of the invention ispolyethylene although other suitable plastics may be used.

[0076] The plastic strip 11 has a base portion 12 with a substantiallyflat side 14. A plurality of length-wise extending rib portions 20projects upward from the base portion 12. In this embodiment, each ribportion 20 comprises a pair of parallel walls 22 and 24 that extendlength-wise along the base portion 12 to define a length wise extendingslot 23. The slot 23 is sized and shaped to snugly receive thereinforcing strip 30 as shown best in FIG. 2.

[0077] A bead of plastics 40 is positioned to bridge the gap between theupper ends of the rib walls 22 and 24 and thereby en-capsulate thereinforcing strip 30 completely. This prevents exposure of thereinforcing strip 30 to the environment and therefore assists inpreventing corrosion.

[0078] In the first embodiment of the invention, an array of threelengthwise extending rib portions 20 spaced apart across the width ofthe strip are provided. Each rib portion 20 supports a correspondingelongate planar metal reinforcing strip 30. In other embodiments of theinvention, more or less ribs and reinforcing strips may be used. Theribs 20 that support the elongate metal reinforcing strips 30 need notbe continuous. The ribs 20 can be any shape provided they support thevertically oriented reinforcing strips 30.

[0079] Referring to FIG. 4, a helically wound composite pipe produced byhelically winding the composite strip shown in FIGS. 1, 2 and 3 isshown. The joint between adjacent edges 18 and 16 of adjacentconvolutions of the strip is best seen in cross-section in FIG. 1.

[0080] Comparing FIGS. 1 and 4, it is apparent that the orientation ofthe reinforcing strips 30 with respect to the flat side 14 of the baseportion 12 remains substantially unchanged after the winding of thestrip to form the pipe. The rib portions 20 provide support for thereinforcing strips 30 particularly during the winding of the strip 10.During winding of the strip 10 to form a helical pipe, the reinforcingstrips 30 are bent about an axis substantially transverse to the strip10. This causes plastic deformation of the reinforcing strips 30. Therib portions 20 assist in preventing the reinforcing strips 30collapsing sideways and towards the base of the plastic strip 12.

[0081]FIG. 5 shows an arc-shaped portion of a reinforcing member 30after it has been bent to wind the pipe shown in FIG. 4. Small areas ofbuckling 32 are illustrated.

[0082] It is important that these areas of buckling are eithernon-existent or relatively small. If excessive buckling is present, theability of the pipe to withstand radial crushing loads is compromised.

[0083] It is also important to keep the mass of the profile to a minimumwhile at the same time maintaining the performance criteria to ensurematerial costs are minimised.

[0084] The dimensions and shapes of the plastic strip 12 and theelongate metal reinforcing strips 30 can be varied to suit the diameterof the pipe to be wound. The below table shows a range of configurationssuitable for pipes of internal diameter ranging from 300 to 600millimeters. Internal Steel Steel Height to Diameter Thickness HeightThickness No. of Steel Steel mm Mm mm Ratio bands Material 150 0.6 46.7:1  3 CA3 SNG 300 0.6 12 20:1 3 CA3 SNG 375 0.6 12 20:1 3 CA3 SNG 4500.6 14 23:1 3 CA3 SNG 525 0.6 16 27:1 3 CA3 SNG 600 0.8 16 20:1 3 CA3SNG 675 1.0 16 16:1 3 CA3 SNG 750 1.2 16 13:1 3 CA3 SNG 825 1.6 16 10:13 CA3 SNG 900 1.6 16 10:1 3 CA3 SNG 1050 1 19 19:1 3 CA3 SNG 1200 1.2 1916:1 3 CA3 SNG

[0085] The height, thickness and number of steel reinforcing bands usedare variables that influence the stiffness of the wound pipe. With pipesof larger diameter, the contribution of the plastic to the stiffness ofthe pipe is relatively small (<10%). With pipes of smaller diameter thecontribution of the plastic to the stiffness of the pipe is higher(approximately 30% for a pipe having internal diameter of 300 mm).

[0086] The height to thickness ratio of the reinforcing strips 30 areimportant for a number of reasons. Reinforcing strips having a highheight to thickness ratio are preferable from the point of view of pipestiffness and efficient use of material but this must be weighed againstthe instability that may result. Instability may cause the reinforcingstrips 30 to collapse sideways towards the base of the plastic strip 12or may cause excessive buckling (buckling is illustrated in FIG. 5).

[0087] Selection of a steel with the optimum Young's modulus (or tensilemodulus) and yield strength for this application is also important.Where yield strength is excessive, buckling is more likely.

[0088] With the range of profiles described in the above table, and witha rib portion thickness range of 1.4 to 1.8 mm, pipes can be wound thatare stable of relatively low weight and have excellent resistance toradial crushing loads.

[0089] Although the embodiment described above uses steel reinforcing,elongate planar reinforcing strips constructed from other materials maybe used.

[0090] The addition of the reinforcing strips 30 to the plastic strip 12can also assist in improving the pressure rating of the pipe. Thecomposite strips described above can further incorporate other elementsto improve the pressure rating of the wound pipe. For instance, laminaof fibre fabric (eg glass fibre), plastic or steel may be provided toimprove the pressure rating of the pipe. Any material having a Young'smodulus and strength that exceeds that of the plastics material of thestrip can be used. The lamina may be incorporated into the profile(strip 12) in any suitable way. For instance, the lamina may be weldedto the base of the strip 12 or may be cross-head extruded into the baseof the strip 12.

[0091] Improved interlocking edge features may also be provided toenhance the pressure rating of the pipe. Examples of profilesconstructed for high-pressure applications are shown in FIGS. 7 throughto 13.

[0092] Referring to FIG. 7, a second embodiment of the invention isshown where the composite strip 10 is extruded from PVC. A mechanicallock is provided by a male edge member 16 and a female edge member 18formed from the plastic strip 11. Reinforcing strips 30 of the typedescribed above are also provided. This profile is cross head extrudedencapsulating the reinforcing strips 30 as the composite strip 10 isproduced obviating the need to add a sealing bead as previouslydescribed. A lamina 50 is incorporated into the base portion of thestrip 11. The lamina 50 has a higher Young's modulus and strength thanthe PVC plastic strip 11. When wound into a helical pipe, this profilecan provide a high pressure pipe suitable for conveying fluids underpressure. Although adjacent convolutions are not directly boundtogether, the thickness of the plastic and design of the mechanical lockformed by adjacent edges 16 and 18 ensures that the pipe is able towithstand significant internal pressures.

[0093]FIG. 8 shows a cross sectional view of two adjacent convolutionsof composite strip 10 according to a third embodiment of the invention.This composite strip 10 comprises a polyethylene extruded strip 11having three rib portions 20 extending from a base portion 12, each ribportion 20 supporting a reinforcing strip 30. A fourth rib portion 21supporting a fourth reinforcing member 31 is also provided. The locationof the fourth rib 21 and reinforcing strip 31 is at the edge of theprofile to strengthen the wound pipe along the gap between the lamina ofadjacent convolutions. This gap 54, is shown in FIG. 8.

[0094] By providing reinforcement on top of the lock between adjacentconvolutions of the composite strip and over the region where the laminais discontinuous, a pipe capable of withstanding high pressure can beproduced.

[0095] A fourth embodiment of the invention is shown in FIG. 9. Thisembodiment of the invention is similar to the third embodiment of theinvention except instead of providing an additional rib and reinforcingmember over the joint area, the female lock section has a thick wall toprovide the pressure capacity where the lamina is discontinuous.

[0096] A fifth embodiment of the invention is shown in FIG. 10 where noadditional features are provided between adjacent convolutions to coverthe area in which the laminae are discontinuous.

[0097] A sixth embodiment of the invention is shown in FIG. 11. Withthis embodiment of the invention, an additional lamina is welded to theedge section of the profile as illustrated.

[0098] A seventh embodiment of the invention is shown in FIG. 12. Thisembodiment of the invention differs slightly to the above-describedembodiment in that the additional lamina 55 is inserted during the pipewinding process.

[0099] A final embodiment of the invention is shown in FIG. 13. Withthis embodiment of the invention, a continuous lamina is either crosshead extruded in the profile base 12 and edge locking areas or is weldedto the base after extrusion.

[0100] Other embodiments of the invention may be provided with thelamina either bonded to the base of the strip 12 or embedded within thebase of the strip 12.

[0101] Materials having directional properties may be used as or withinthe lamina. For instance, orientated plastic film strips that are strongin a longitudinal direction and weak in a transverse direction may beused. Such strips may improve the “hoop” strength of the wound pipe.

[0102] Plastic film strips that are strong in a transverse direction andweak in a longitudinal direction may also be used.

[0103] In some applications it will be desirable to form a lamina fromtwo (or more) plastic film strips that are strong in mutually orthogonaldirections thereby resulting in a composite of high strength in alldirections.

[0104] Examples of suitable materials having directional propertiesinclude highly stretched polyolefin sheet. Such sheets have a highproportion of molecules orientated in the same direction which providesa high Young's modulus and yield strength.

[0105] Currently, helically wound composite pipes are formed inmulti-stage operations. Generally a plastics body is extruded within afactory environment and is then wound onto a spool for transport. Theextruded strip is then unwound from the spool and passed through awinding machine that can also be located within a factory oralternatively can be located at the site where the final pipe isrequired. Finally, elongate steel reinforcing strips are rolled onto thenewly wound pipe. In many applications, the steel reinforcing strips arepre-rolled to a radius approximating that of the helically woundplastics body before they are introduced onto the outside of theplastics pipe to form a composite pipe with a requisite stiffness. Thepre-bending of the reinforcing strip is required where reinforcingmembers have a high degree of stiffness through the relevant bendingaxis.

[0106] The process of forming a helical pipe from the profile describedabove with reference to FIGS. 1, 2, 3, 5 and 6 is simplified since thereinforcing members 30 are introduced into the strip at an early stageof manufacture and before the pipe is wound.

[0107] A method of constructing a composite strip 10 windable to form ahelical pipe is shown in FIG. 6. A plastics strip 11 is extruded havinga substantially flat sided base portion and an array of parallel spacedapart length wise extending rib portions 20 upstanding from the baseportion 12. Next a plurality of elongate metal reinforcing strips 30 areintroduced the rib portions 20. The reinforcing strip 30 has a height tothickness ratio of at least four to one and is orientated substantiallyperpendicular to the flat side 14 of the base portion 12.

[0108] The introducing or inserting step described above occurs whilethe plastic strip is lying substantially flat. The reinforcing strips 30are inserted straight without any pre-bending. Finally beads of plastic40 (as shown in FIGS. 1 and 2) are extruded onto the tops of the ribportions 20 to encapsulate the reinforcing strips 30.

[0109] A further method of constructing a composite strip windable toform a helical pipe is as follows. Plastics material and steel strip areintroduced into an extrusion cross-head die where the two materials areintegrated into one composite profile, such as the composite stripdescribed above and shown in FIG. 3. A composite strip formed bycross-head extrusion may differ slightly from the profile describedabove in that the beads of plastic 40 (as shown in FIGS. 1 and 2) wouldnot be required instead, the cross-head extrusion die could be designedsuch that the steel strip exits the die completely encapsulated withplastics material.

[0110] Having produced a composite reinforced strip, it is possible todirectly wind that strip into a helically wound pipe such as the pipeshown in FIG. 4 or alternatively, the strip can be rolled onto a spoolfor later use.

[0111] The ability to roll the composite profile onto a spool fortransport provides a number of advantages. For instance, a single spoolcan be transported to the field and positioned adjacent a pipe windingmachine located where the final pipe is required. Composite pipe canthen be helically wound in a single operation without the need for largeamounts of specialised equipment.

[0112] In order to be able to spool the straight composite strip 12withut the steel reinforcing strips 30 buckling it was necessary todevelop a new method of spooling. Existing conventional spooling methodscreate a strip path that reverse bends the strip and then straightens itprior to the strip going onto the hub of the spool. The spool is rotatedabout a horizontal axis with the strip being fed to the top or uppersideof the spool. For plastic strips without steel this method issatisfactory. However, when there is steel reinforcing in the strip,this method is not suitable as it causes the steel reinforcing 30 tobuckle.

[0113]FIG. 14 shows a spool drive assembly 100 developed for spoolingsteel reinforced composite strip 10. The spool 101 is supported forrotation about a horizontal axis 102. A strip guide 110 is provided todistribute the strip 10 across the width of the spool's hub. An endlesspneumatic cylinder 114 driving on a rod 112 drives the strip guide 110back and forth.

[0114] The spooling method developed for steel reinforced strip andshown in FIG. 14 has a strip path that minimises any load applied to thestrip that could cause buckling. The strip path onto the spool 101 withthis arrangement is a straight path to the bottom or underside 103 ofthe spool with the ribs facing down and hence the base portion 12 facingup allowing the strip to be bent in the correct orientation on the spool(ribs facing outwards, as they do in the wound pipe).

[0115] The method of controlling the speed of rotation of the spool 101developed for this new method relies on the tension in the strip 10(torque on the motor). In addition to changing the spooling methods, theoptimum spool hub size needs to be selected to prevent the ribs bucklingduring the spooling process. An initial hub size of 450 mm was trialedwhich was suitable for some of the steel thicknesses, however as thesteel becomes thicker and taller the hub size needs to be increased. Forthe current strip 10 made for pipes up to 750 mm in diameter, a hub sizeof 1000 mm is required.

[0116] The profiles of the second to eighth embodiments of the inventionas illustrated in FIGS. 7 to 13, can be constructed using the methoddescribed above for the profile of the first embodiment of the inventionas shown in FIGS. 1 to 6. The lamina can be introduced in a separatestep after the strip has been extruded.

[0117] While the present invention has been described in terms of apreferred embodiment in order to facilitate better understanding of theinvention, it should be appreciated that various modifications can bemade without departing from the principles of the invention. Therefore,the invention should be understood to include all such modificationswithin its scope.

The claims defining the invention are as follows:
 1. A composite stripwindable to form a helical pipe, the composite strip comprising: anelongate plastic strip having a base portion and at least one lengthwiseextending rib portion upstanding from the base portion; and an elongatereinforcing strip extending lengthwise and supported laterally by therib portion, the reinforcing strip having a height to thickness ratio ofat least three to one and orientated substantially perpendicular to thebase portion, wherein, when wound into a helical pipe, the reinforcingstrip reinforces the pipe against radial crushing loads.
 2. A compositestrip as claimed in claim 1 wherein said height to thickness ratio is atleast four to one.
 3. A composite strip as claimed in either of claims 1or 2 wherein the rib portion defines a lengthwise extending slot inwhich the reinforcing strip is retained, the strip being laterallysupported by the walls of the slot.
 4. A composite strip as claimed inclaim 3 wherein the rib portion comprises a pair of parallel wallsextending lengthwise along the base portion, the parallel wallsorientated substantially perpendicular to the base portion.
 5. Acomposite strip as claimed in claim 4 wherein the reinforcing strip iscontinuous and has a length which is co-extensive with the plasticstrip.
 6. A composite strip as claimed in claim 5 wherein thereinforcing strip is completely encapsulated so as to prevent exposureto the environment.
 7. A composite strip as claimed in claim 6 whereinthe plastic strip has an array of lengthwise extending slot forming ribportions spaced apart across the width of the strip, each rib portionsupporting an elongate reinforcing strip.
 8. A composite strip asclaimed in any one of claims 1 to 7 wherein the reinforcing strip isconstructed from metal.
 9. A composite strip as claimed in claim 8wherein the reinforcing strip is constructed from steel.
 10. A compositestrip windable to form a helical pipe, the composite strip comprising:an elongate plastic strip having a base portion and a lengthwiseextending rib portion upstanding from the base portion; an elongateplanar reinforcing strip extending lengthwise and supported by the ribportion, the reinforcing strip having a height to thickness ratio of atleast three to one and orientated substantially perpendicular to thebase portion; and a planar lamina extending lengthwise and bonded to thebase portion, the lamina having a higher Young's modulus and strengththan those of the plastic strip, wherein, when wound into a helicalpipe, the reinforcing strip reinforces the pipe against radial crushingloads and the lamina improves the pressure rating of the pipe.
 11. Acomposite strip as claimed in claim 10 wherein said height to thicknessratio is at least four to one.
 12. A composite strip as claimed ineither of claims 10 or 11 wherein the rib portion defines a lengthwiseextending slot in which the reinforcing strip is retained, the stripbeing laterally supported by the walls of the slot.
 13. A compositestrip as claimed in claim 12 wherein the rib portion comprises a pair ofparallel walls extending lengthwise along the base portion, the wallsorientated substantially perpendicular to the flat side.
 14. A helicallywound composite pipe produced from a composite strip, the compositestrip comprising: an elongate plastic strip having a base portion thatforms the inside of the wound pipe and a lengthwise extending ribportion upstanding from the base portion; and an elongate metalreinforcing strip extending lengthwise and supported by the rib portion,the reinforcing strip having a height to thickness ratio of at leastthree to one and orientated substantially perpendicular to the baseportion, wherein the orientation of the reinforcing strip with respectto the base portion remains substantially unchanged after the winding ofthe strip to form the pipe.
 15. A pipe as claimed in claim 14 whereinsaid height to thickness ratio is at least four to one.
 16. A pipe asclaimed in either of claims 14 or 15 wherein the reinforcing strip iscontinuous and has a length which is co-extensive with the pipe.
 17. Apipe as claimed in any one of claims 14 to 16 wherein the reinforcingstrip is constructed from metal.
 18. A pipe as claimed in claim 17wherein the reinforcing strip is constructed from steel.
 19. A helicallywound composite pipe produced from a composite strip, the compositestrip comprising: an elongate plastic strip having a base portion thatforms the inside of the wound pipe and a lengthwise extending ribportion upstanding from the base portion; an elongate planar reinforcingstrip extending lengthwise and supported by the rib portion, thereinforcing strip having a height to thickness ratio of at least threeto one and orientated substantially perpendicular to the base portion;and an elongate planar lamina extending lengthwise and bonded to thebase portion, the lamina having a higher Young's modulus and strengththan those of the plastic strip, wherein the orientation of thereinforcing strip with respect to the base portion remains substantiallyunchanged after the winding of the strip to form the pipe.
 20. A pipe asclaimed in claim 19 wherein adjacent convolutions of the lamina are notdirectly bonded together.
 21. A pipe as claimed in claim 20 wherein saidheight to thickness ratio is at least four to one.
 22. A pipe as claimedin either of claims 20 or 21 wherein the reinforcing strip is continuousand has a length which is co-extensive with the pipe.
 23. A pipe asclaimed in any one of claims 19 to 22 wherein the reinforcing strip isconstructed from metal.
 24. A pipe as claimed in claim 23 wherein thereinforcing strip is constructed from steel.
 25. A helically woundcomposite pipe produced from a composite strip, the composite stripcomprising: an elongate plastic strip having a base portion that formsthe inside of the wound pipe and a lengthwise extending rib portionupstanding from the base portion; an elongate reinforcing stripextending adjacent to or integral with the rib portion; and an elongateplanar lamina extending lengthwise and bonded to the base portion, thelamina having a higher Young's modulus and strength than those of theplastic strip.
 26. A pipe as claimed in claim 25 wherein adjacent woundconvolutions of the lamina of the pipe are not directly bonded together.27. A pipe as claimed in claim 26 wherein the reinforcing strip isconstructed from metal.
 28. A method of producing a helically woundsteel reinforced plastics pipe comprising the steps of: extruding aplastics profile having a base portion and a lengthwise extending ribportion upstanding from the base portion; introducing an elongatestraight edged reinforcing strip into the rib portion, the metal striphaving a height to thickness ratio of at least three to one andorientated substantially perpendicular to the base portion, therebyproducing a straight composite strip; helically winding the compositestrip; and inter-engaging the adjacent edges of adjacent convolutions ofthe strip so as to form a helical pipe.
 29. A method as claimed in claim28 wherein the rib portion is extruded to define a lengthwise extendingslot shaped support the reinforcing strip.
 30. A method as claimed inclaim 29 wherein the rib portion is extruded to comprise a pair ofparallel walls extending lengthwise along the base portion, the wallsorientated substantially perpendicular to the base.
 31. A method asclaimed in claim 30 further comprising the step of encapsulating thereinforcing strip.
 32. A method as claimed in claim 31 wherein theextruding and introducing steps occur together in a cross-head extrusiondie.
 33. A method of producing a helically wound steel reinforcedplastics pipe comprising the steps of: extruding a plastics profilehaving a base portion and a lengthwise extending rib portion upstandingfrom the base portion; and introducing an elongate metal reinforcingstrip into the rib portion, the metal strip having a height to thicknessratio of at least three to one and orientated substantiallyperpendicular to the base portion, thereby producing a composite strip;bonding a lamina to the base portion, the lamina having a higher Young'smodulus and strength than those of plastic strip; helically winding thecomposite strip; and inter-engaging the adjacent edges of adjacentconvolutions of the strip so as to form a helical pipe.
 34. A method asclaimed in claim 33 wherein the rib portion is extruded to define alengthwise extending slot shaped support the reinforcing strip.
 35. Amethod as claimed in claim 34 wherein the rib portion is extruded tocomprise a pair of parallel walls extending lengthwise along the baseportion, the walls orientated substantially perpendicular to the base.36. A method according to claim 28 having further steps between theintroducing and helically winding steps, the further steps comprising:directing the straight composite step to a spool having a hub rotatingabout a substantially horizontal axis, with the base portion of thestrip facing the underside of the hub; driving the spool so as to pullthe straight composite strip towards the spool and so as to wind thestrip around the hub of the spool from its underside; transporting thespool to a site; and unwinding the strip from the spool.
 37. A methodaccording to claim 33 having further steps between the bonding andhelically winding steps, the further steps comprising: directing thestraight composite strip to a spool having a hub rotating about asubstantially horizontal axis, with the base portion of the strip facingthe underside of the hub; driving the spool so as to pull the straightcomposite strip towards the spool and so as to wind the strip around thehub of the spool from its underside; transporting the spool to a site;and unwinding the strip from the spool.